As microprocessor technology continues to evolve, the corresponding scale size of computer chips continues to shrink. Silicon based transistors are being manufactured smaller and smaller, however this meets the problem of increased power dissipation, especially power dissipated in the form of heat. Historically, to counter this problem, microprocessor heat is transferred and distributed using fans and heat sink structures.
Traditionally, the current approach for cooling is to conduct heat away from a silicon microprocessor as a whole. FIG. 1 displays a heat sink attached to a microprocessor. A silicon microprocessor (10) is fixed between a substrate layer (12) and a first thermal interface material (TIM) layer (14). A lid (18) encases the silicon microprocessor (10), abutting the first TIM layer (14), and fixed to the top of the substrate layer (12). Fixed to the top of the lid is a second TIM layer (16). Fixed above the second TIM layer (16) is a heat sink structure (20) comprising a plurality of fins configured with a large surface area to distribute heat. As heat is dissipated from the silicon microprocessor, the first (14) and second (16) TIM layers and lid (18), which are made of materials chosen for their high thermal conductivity, transfer the heat to the heat sink structure (20).
Thermal interface material (TIM) is typically placed between microprocessors and heat sinks to increase thermal transfer between the two devices. The microprocessor and heat sink device are solid and, in practice, tend not to have a perfectly flat ideal surface. When abutting the heat sink device directly to the lid or microprocessor, there will likely be air gaps due to portions where the surfaces are not flat and smooth. A TIM layer is placed between the microprocessor and the heat sink to fill in these air gaps. This allows for much higher thermal heat transfer.
The TIM layer can take on many forms, the most common perhaps, is known as “thermal grease.” Thermal grease is typically silicone oil filled with aluminum oxide, zinc oxide, or boron nitride. Alternatively, pulverized silver may be used. A grease or oil-like substance provides a good interface between the two solid objects; air gaps are filled and high heat transfer can be achieved.
An additional type of thermal interface material is known as phase change material. At room temperature, these are solid materials, however, they behave as a grease-like substance at the processor's hotter operating temperatures.
Heat pipes are well known for numerous cooling applications from CPUs to nuclear power cells. See, for example, U.S. Pat. No. 6,889,755. Heat pipes employ a technique known as evaporative cooling. Referring to FIG. 2, a heat pipe (36) comprises a vacuum tight envelope (30), a wick structure (34), and a working fluid (32). Typically, the heat pipe is evacuated and then filled with a small amount of working fluid (32), just enough to saturate the wick (34). In practice, the wick (34) is often made of a porous material and configured to line the internal wall of the heat pipe (36). An equilibrium of liquid and vapor sets the atmosphere inside the heat pipe. The equilibrium is upset as heat enters the evaporator end (35), generating vapor at a slightly higher pressure. This higher pressure vapor travels up to the condenser end (37). Due to slightly lower temperatures at the condenser end (37), the vapor condenses, giving up its latent heat of vaporization. The condensed fluid is then drawn back to the evaporator end (35) by the capillary forces developed in the wick structure (34). Heat pipes, over other conducting structures, transfer heat from the evaporator end to the condenser end very well.
Another cooling technique uses diamond material to conduct heat away from silicon microprocessors. Diamond has been known as a good conductor of heat because of the strong covalent bonding within the crystal. Most natural blue diamonds contain boron atoms which replace carbon atoms in the crystal matrix, and also have high thermal conductivity. Because diamond has such high thermal conductance, it is used in semiconductor manufacture to prevent silicon and other semiconductor materials from overheating. Diamond conductors can be manufactured synthetically, however, they are rather expensive.